Metallic powders or mixtures are generally fashioned into solid forms by first forming them into a die cavity and applying pressure to form a compound. In a separate operation, the compact is heated and forms a coherent mass. Optionally, the uncompacted powder can be heated without first applying pressure.
The shape and size of the cavity may be exactly as required by the finished part or may be some intermediate configuration. The pressing operation is usually carried out at room temperature, however, warm or even hot pressing can be employed. This pressing operation is used to consolidate the powder into a mass which can be handled and which has a certain dimensional configuration. During pressing the coherent mass is formed through the processes of interparticle binding and interlocking. Application of heat during pressing, or as a separate step, is such as not to cause melting of the powder. The temperature to which the pressed compact has been heated is below, and usually very close to, the melting point of the metal as it is elemental, or aqueous temperature of any alloy which is formed.
In many applications porous solids are needed. Metallic foams are one example of such porous solids. Their internal structure is one of a plurality of randomly dispersed closed cells throughout a metal matrix. Limited porosity is achieved. One method of foam production is to employ a heat decomposable foaming agent to generate gas and form the cells. Such foaming techniques frequently produce closed cells which are non-uniform or are undesirably too large. These problems have been addressed to some extent by increasing the viscosity of the molten metal of the various viscosity-increasing agents to aid in a subsequent flowing step.
U.S. Pat. No. 3,816,952, dated June 18, 1983, to Niebylski et al, provides a method for producing an aluminum base foam which comprises the following steps: increasing the viscosity of a molten aluminum-base metal with a viscosity-increasing amount of a viscosity-increasing agent; and, treating the viscous melt thereby produced. Oxygen is mentioned as one of the preferred foaming agents. The urea system produced is heated sufficiently to thermally decompose the flowing agent to release gas which makes the foaming take place (formation of closed cells). Porosity arises within the interstices between the closed cells.
U.S. Pat. No. 3,669,654, dated June 13, 1972, to Barry, Jr., discloses the process of foam production where a metal or alloy is melted and a viscosity-strength increasing agent added. The agent must contain oxygen and form oxide particles in situ. This composition is thinned and brought to a proper temperature before the blowing agent is introduced.
Metallic bodies, including foams, disclosed to date lack a uniform porosity coupled with high surface area and minimum flow resistance. By this is meant gas flows through the solidified metallic body. A porous metallic body having low density, high surface area, and an interconnecting porosity structure sufficient to provide the flow-through of gas through the body is highly desirable. It would be an advancement in the art to provide such a porous metallic body possessing these characteristics.